The present disclosure relates to magnetic resonance imaging (MRI) systems.
MRI systems are used to investigate the anatomy and physiology of the human body. The physics of MRI systems involves the interaction of matter with electromagnetic fields. The tissue of the human body is largely composed of water molecules, each containing two hydrogen nuclei, or protons. When inside the static magnetic field (B0) of an MRI system, the magnetic moments of these protons align with the direction of the static magnetic field. A radio frequency pulse can be transmitted by a coil into the body, causing the protons to alter their magnetization alignment relative to the static magnetic field. In response to the force bringing them back to their equilibrium orientation, the protons undergo a rotating motion (precession). These changes in magnetization alignment cause a changing magnetic flux, which can be sensed as a time varying voltage signal in the coil. The frequency at which a proton or group of protons in a voxel of the body resonates depends on the strength of the local magnetic field around the proton or group of protons. By applying additional magnetic fields (gradients), specific slices to be imaged can be selected and an image can be obtained.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates (i.e., they have different relaxation times). By changing the operating parameters of the MRI system, this effect can be used to create contrast between different types of body tissue. Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike computerized tomography, MRI system do not use ionizing radiation and are generally very safe. MRI systems can be used to image every part of the human body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumors, and for showing abnormalities in the heart and blood vessels.
In an MRI system, resonant absorption of energy by a proton due to an external oscillating magnetic field will occur at the Larmor frequency for the proton. The spin of the proton has two states. The net longitudinal magnetization in thermodynamic equilibrium is due to a tiny excess of protons in the lower energy state. This gives a net polarization that is parallel to the external oscillating magnetic field. Typically, the magnetic field is caused to vary across the body of the subject by using a field gradient, so that different spatial locations become associated with different precession frequencies. Usually these field gradients are pulsed. The variety of RF and gradient pulse sequences that can be used gives MRI systems their versatility. Application of a radio frequency (RF) pulse can tip this net polarization vector sideways (with, i.e., a so-called 90° pulse), or even reverse it (with a so-called 180° pulse). The recovery of longitudinal magnetization is called longitudinal or T1 relaxation and occurs exponentially with a time constant T1. The loss of phase coherence in the transverse plane is called transverse or T2 relaxation and occurs with a T2 time constant.
A number of MRI schemes have been devised for combining field gradients and radio frequency excitation to create an image. These MRI schemes include 2D or 3D reconstruction from projections (such as in computed tomography), and building the image point-by-point or line-by-line. These MRI schemes employ pulsed field gradients. The pulsed field gradients can be used in two ways: frequency encoding and phase encoding. The use of pulsed field gradients for frequency encoding is accomplished by acquiring a signal with the pulsed field gradient active (ON). In this case, the frequency of the signal is directly related to the location of the signal. The use of pulsed field gradients for phase encoding is accomplished by acquiring a signal after termination of a pulsed field gradient. In this case, the additional phase to the signal, referred to as ϕ, can be equated to the product γ*g*δ, where γ is the gyromagnetic ratio of the nuclei detected (often hydrogen, 4257 Hz/G), g is the gradient (unit G/cm), and δ is the turn-on period of the pulses field gradient (often called pulse length). By multiple acquisition of the signal with different values of the phase ϕ, the signal dependence on the phase ϕ is obtained. This dependence can be used to obtain an image typically via Fourier transform techniques.
These MRI schemes can also use slice selection where the signal from a slice of the measurement space is selected and detected. This is accomplished by transmitting an RF pulse while the field gradient is active (ON). With the field gradient active, the magnetic field within the gradient coil exhibits a linear variation. The RF pulse will then only excite the signals from the position where the frequency of the RF pulse, referred to as f, matches the product of γ and the effective magnetic field B, i.e., the product γ*B, as follows:f˜γ*B.  (1)The effective magnetic field B is produced by the magnet and the gradient coil and can be given as:B=B0+g*x,  (2)                where B0 is the static magnetic field produced by the magnet,                    g is the field gradient (in units of G/cm) produced by the gradient coil, and            x is the spatial coordinate (in units of cm).Note that the contribution of the field gradient g (and its frequency) varies linearly with respect to the coordinate x. Plugging Eqn. (2) into Eqn. (1) gives:f˜γ*(B0+g*x), which can be written as  (3a)f˜γB0+γ(g*x).  (3b)                        
With the field gradient g applied along a particular direction (e.g., along the axis for coordinate x), the slice selection plane corresponds to the plane perpendicular to the direction of the field gradient, e.g., the y-z plane in this example. If the field gradient g is applied along the z direction, the slice selection plane will correspond to the x-y plane.
It is also possible to produce other field gradients. For example, a permanent magnet array can be used to produce a constant field gradient whose area will be a sheet which can be curved dependent on the details of the magnet array. In another example, slice selection phase encoding can employ an inhomogeneous field as described in Blümich et al., “Mobile single-sided NMR,” Progress in Nuclear Magnetic Resonance Spectroscopy, Vol. 52(4), 2008, pgs. 197-269.
It is also possible to use slice selection with a CPMG pulse sequence to perform projections at different angles. This method is based on project reconstruction (Radon transform) and the inversion requires straight line projections as described in U.S. Patent Application Publication No. 2013/0176026 to Y Song, Fei Han, and Jeff Paulsen.
A typical MRI system includes a large magnet shaped like a cylinder. The inner opening of the cylindrical-like permanent magnet typically has a diameter of 40-60 inches to allow a person to lie inside. The outside diameter of the cylindrical-like permanent magnet is typically about 2 meters. The weight of such system is typically over 1 ton. In addition, the magnet is typically a superconducting magnet that requires cryogenic cooling (liquid nitrogen and helium). Furthermore, RF coils are typically imbedded in the magnet cavity with the corresponding diameter of 40 inches or larger. Because of the large volume, the power needed to energize such RF coils is several kilowatts. In addition, the system employs pulsed gradient coils that require many kilowatts of power for each gradient (typically 3). Thus, an MRI system is always large, expensive in the equipment cost, installation and operation. Most of such systems are permanent installation. The so-called mobile systems are based on permanent magnet and installed in large tractor-trailer and weigh several tons.
Smaller MRI systems have been developed for the imaging of extremities, such as wrist, ankles, and foot. They are an essentially smaller version of the full-body scanners and the technologies and equipment used are essentially the same as the larger counterpart.
All of the MRI systems commercially-available today, including full-body system and extremity systems, employ RF pulses in combinations with pulsed field gradients. They operate to acquire images with stationary equipment (magnet, coils) relative to the subject. Multiple acquisitions are obtained and then an image is formed based on the data.